Most headphones are of so-called closed type. The headphones of this type is an acoustic apparatus configured to produce optimal sound in closed spaces between headphone units and ears. In contrast, a speaker is designed to be used for listening to sound at some distance. The headphones and the speaker are structurally similar. It can be said that the most important difference therebetween is power (sound pressure) for vibrating air. The headphones only need to have power for vibrating air in the closed spaces. On the other hand, the speaker needs high power for vibrating air in a space between the speaker and the user.
Patent Document 1 has disclosed headphones in which the output power of a headphone/speaker is switched between a low level for the headphone and a high level for the speaker by a changeover switch operation. Thus, the headphones have both of headphone and speaker functions.
However, the headphones of Patent Document 1 merely switch the output power of the speaker unit placed in a housing between the low level for the headphone and the high level for the speaker. Therefore, for using the speaker unit as the speaker, it has to be used as a stationary speaker. At this time, a distance between speaker units respectively placed in left and right housings is not sufficient for providing a sufficient stereophonic effect in stereophonic reproduction.
Patent Document 1: Japanese Patent Application Laid-open No. 2010-74831 (paragraph [0019], etc.)